Question

我是gson的新手，正在尝试解决如何对对象中的列表进行反序列化。

错误消息提示创建InstanceCreator for Player，我这样做了。但是在实现时，我发现反序列化对象包含一个名称字段设置为“默认值”的玩家列表，而不是从json字符串中获取值。所以我现在想知道这种方法是否正确。

这是我正在处理的模型的简化，但突出了问题......

public interface Player {
    String name();
}

public class PlayerImpl implements Player {

    private String name;

    public PlayerImpl(String name) {
        this.name = name;
    }

    public String name() { return this.name; }
}

public interface Team {
...
}

public class TeamImpl implements Team {

    String name;
    List<Player> players = new ArrayList<>();

    public TeamImpl(String teamName) { this.name = teamName; }

    ...
}

我有一个简单的测试来创建一个有2个玩家的新团队

Team t = new TeamImpl("teamname");
t.addPlayer(new PlayerImpl("p1"));
t.addPlayer(new PlayerImpl("p2"));
Gson gson = new Gson();
String json = gson.toJson(t);

创建以下json字符串：

{"name":"teamname","players":[{"name":"p1"},{"name":"p2"}]}

然而，当我反序列化json字符串...

Team t2 = gson.fromJson(json, TeamImpl.class);

我收到以下错误：

java.lang.RuntimeException: Unable to invoke no-args constructor for interface com.example.Player. Register an InstanceCreator with Gson for this type may fix this problem.

    at com.google.gson.internal.ConstructorConstructor$14.construct(ConstructorConstructor.java:226)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:210)
    at com.google.gson.internal.bind.TypeAdapterRuntimeTypeWrapper.read(TypeAdapterRuntimeTypeWrapper.java:41)
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:82)
    at com.google.gson.internal.bind.CollectionTypeAdapterFactory$Adapter.read(CollectionTypeAdapterFactory.java:61)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:129)
    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:220)
    at com.google.gson.Gson.fromJson(Gson.java:887)
    at com.google.gson.Gson.fromJson(Gson.java:852)
    at com.google.gson.Gson.fromJson(Gson.java:801)
    at com.google.gson.Gson.fromJson(Gson.java:773)
    at com.example.data.JsonDataTests.test_team_json(JsonDataTests.java:62)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at org.junit.internal.runners.TestMethodRunner.executeMethodBody(TestMethodRunner.java:99)
    at org.junit.internal.runners.TestMethodRunner.runUnprotected(TestMethodRunner.java:81)
    at org.junit.internal.runners.BeforeAndAfterRunner.runProtected(BeforeAndAfterRunner.java:34)
    at org.junit.internal.runners.TestMethodRunner.runMethod(TestMethodRunner.java:75)
    at org.junit.internal.runners.TestMethodRunner.run(TestMethodRunner.java:45)
    at org.junit.internal.runners.TestClassMethodsRunner.invokeTestMethod(TestClassMethodsRunner.java:71)
    at org.junit.internal.runners.TestClassMethodsRunner.run(TestClassMethodsRunner.java:35)
    at org.junit.internal.runners.TestClassRunner$1.runUnprotected(TestClassRunner.java:42)
    at org.junit.internal.runners.BeforeAndAfterRunner.runProtected(BeforeAndAfterRunner.java:34)
    at org.junit.internal.runners.TestClassRunner.run(TestClassRunner.java:52)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:121)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
    at com.intellij.rt.execution.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:51)
    at com.intellij.rt.execution.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:237)
    at com.intellij.rt.execution.junit.JUnitStarter.main(JUnitStarter.java:70)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:497)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)
Caused by: java.lang.UnsupportedOperationException: Interface can't be instantiated! Interface name: com.example.Player
    at com.google.gson.internal.UnsafeAllocator.assertInstantiable(UnsafeAllocator.java:117)
    at com.google.gson.internal.UnsafeAllocator.access$000(UnsafeAllocator.java:31)
    at com.google.gson.internal.UnsafeAllocator$1.newInstance(UnsafeAllocator.java:49)
    at com.google.gson.internal.ConstructorConstructor$14.construct(ConstructorConstructor.java:223)
    ... 35 more

Answer 1

Gson清楚地报告它无法实例化接口。为了使其成为可能，您可以注册一个自定义类型适配器，该适配器将知道如何序列化实例并将其反序列化。

final class InterfaceSerializer<T>
        implements JsonSerializer<T>, JsonDeserializer<T> {

    private final Class<T> implementationClass;

    private InterfaceSerializer(final Class<T> implementationClass) {
        this.implementationClass = implementationClass;
    }

    static <T> InterfaceSerializer<T> interfaceSerializer(final Class<T> implementationClass) {
        return new InterfaceSerializer<>(implementationClass);
    }

    @Override
    public JsonElement serialize(final T value, final Type type, final JsonSerializationContext context) {
        final Type targetType = value != null 
                ? value.getClass() // `type` can be an interface so Gson would not even try to traverse the fields, just pick the implementation class 
                : type;            // if not, then delegate further
        return context.serialize(value, targetType);
    }

    @Override
    public T deserialize(final JsonElement jsonElement, final Type typeOfT, final JsonDeserializationContext context) {
        return context.deserialize(jsonElement, implementationClass);
    }

}

然后配置你的Gson实例（每个应用程序一次，Gson是不可变的和线程安全的）：

Gson gson = new GsonBuilder()
        .registerTypeAdapter(Player.class, interfaceSerializer(PlayerImpl.class))
        .registerTypeAdapter(Team.class, interfaceSerializer(TeamImpl.class))
        .create();

请注意，现在即使Team t2 = gson.fromJson(json, Team.class);也可以使用（不是TeamImpl.class。

也许基于意见，但是......我强烈建议你不要绑定Gson中的接口。 Gson唯一的责任是对您的应用程序使用的业务/价值对象进行序列化和反序列化。查看描述问题的Data Transfer Object模式，并建议对与应用程序进行传输数据的对象进行分段。有了这个，您可以从DTO中删除接口，这样您就不需要这样的适配器，也不关心如何声明和注释DTO类。知道您可能有TeamDto和PlayerDto甚至无法实现您的接口，但清楚地绑定数据，比如class TeamDto { private List<PlayerDto> }。 DTO可以很容易地转换为普通的实现，也可以从它们中轻松构建。

Answer 2

Gson在反序列化期间使用POJO而不是接口。请使用实施PlayerImpl而不是接口Player。还有其他库，如Genson，它们支持您的要求。

Answer 3

你有这个错误：Caused by: java.lang.UnsupportedOperationException: Interface can't be instantiated! Interface name: com.example.Player所以你应该在TeamImpl里面建一个列表，像这样`List players = new ArrayList（）;因为无法在GSON中实例化接口。

public interface Player {
 String name();
}
public class PlayerImpl implements Player {

private String name;

public PlayerImpl(String name) {
    this.name = name;
}

public String name() { return this.name; }
}


public interface Team {

   void addPlayer(PlayerImpl p1);
}


import java.util.ArrayList;
import java.util.List;

public class TeamImpl implements Team {

String name;
List<PlayerImpl> players = new ArrayList<PlayerImpl>();

public TeamImpl(String teamName) { this.name = teamName; }


@Override
public void addPlayer(PlayerImpl p1) {
    this.players.add(p1);
}
}

And the main to test it:
public static void main(String[] args) {

Team t = new TeamImpl("teamname");
t.addPlayer(new PlayerImpl("p1"));
t.addPlayer(new PlayerImpl("p2"));
Gson gson = new Gson();
String json = gson.toJson(t);

System.out.println(json);

Team t2 = gson.fromJson(json, TeamImpl.class);
System.out.println(t2.toString());

}

`

Answer 4

其他答案几乎是正确的，但错过了您提供类而不是fromJson方法的接口的事实：

Team t2 = gson.fromJson(json, TeamImpl.class);

这里的问题是你没有默认构造函数（没有args），因此无法调用Class＃newInstance。

您需要向GSON提供自定义反序列化器，已经回答here

Answer 5

通常，当类使用接口作为字段时，也会引起此异常。尤其是在创建树结构时，很难发现自己在做错什么：

class A implements IA{
   IA parent;
   Collection<IA> nodes;
}

IA myimpl = gson.fromJson( data_str, A.class );

其中IA是类A的接口。此代码将引发上面的异常。

gson - 反序列化包含列表的对象

5 个答案: